In the design of large hole rock cutters, such as raise drills, a plurality of rotary cutters are mounted on a frame in the form of a disk having a drive stem attached at the centr for imparting rotation to the disk. The cutter units are positioned at various radial distances from the axis of rotation. Such a drill is described, for example, in U.S. Pat. No. 3,675,729. The tool is used by first drilling a small pilot hole between the two levels. A drill string is then lowered through the pilot hole from rotary drive equipment at the upper level and a raise drill tool is attached at the lower level to the drill string. The drill is then drawn upwardly while simultaneously being rotated by the drill string, the drill reaming out the hole to the desired diameter. In an exemplary embodiment the pilot hole may be 11 inches diameter and the raise drill reams out a hole 6 feet in diameter. To permit initial installation and replacement of the rotary cutters as they wear, various detachable mounting arrangements have been provided. Typically the cutter is journaled on a shaft or pin, the ends of the pin being anchored to a yoke structure. Various clamping means for releasably securing the ends of the shaft to permit removal of the shaft and the cutter have been proposed. See, for example, U.S. Pat. No. 3,612,196. This mounting arrangement must provide very rigid anchoring to prevent loosening or wear by the severe vibration and shocks to which the drill is subjected.
In the past considerable difficulty has been experienced in the mounting of the inboard cutters in this regard. The inboard cutters must be mounted with the cutters tangent to or slightly recessed into the drive stem in a raise drill to assure reaming adjacent the pilot hole in which the stem travels. As a result, the inboard cutters are usually mounted by inserting the inner end of the journal shaft of the cutter into a radial bore in the drive stem. The other end of the journal shaft is supported from the disk in a manner which allows the journal shaft to be moved axially to permit the shaft to be inserted or withdrawn from the bore in the drive stem.
To permit the shaft to slide in and out of the bore, the bore must be slightly larger in diameter than the shaft. Under normal tolerances, this allows some room for play between the shaft and the bore, resulting in wear and gradual loss of rigidity in the mounting of the inboard cutters under the constant vibration and pounding experienced by the drill. Heretofore there has been no effective way of securely locking the inner end of the journal shaft in the bore to prevent relative movement and wear between the journal shaft and the drive stem.